A CMP (Chemical Mechanical Polishing) apparatus is a machine for polishing a surface of a substrate, such as a wafer, by pressing the substrate against a polishing pad while supplying a polishing liquid onto the polishing pad. The CMP apparatus is widely known as a polishing apparatus for manufacturing a semiconductor device.
FIG. 1 is a schematic view showing a polishing apparatus for polishing a wafer. As shown in FIG. 1, the polishing apparatus includes a polishing table 2 for supporting a polishing pad 1, and a top ring (or a substrate holder) 5 for pressing a wafer W against the polishing pad 1. The polishing table 2 is coupled via a table shaft 2a to a table motor 3 which is located below the polishing table 2, so that the polishing table 2 is rotated by the table motor 3 in a direction indicated by arrow. The polishing pad 1 is attached to an upper surface of the polishing table 2. The polishing pad 1 has an upper surface 1a that serves as a polishing surface for polishing the wafer W. The top ring 5 is secured to a lower end of a top ring shaft 6. The top ring 5 is configured to hold the wafer W on its lower surface by vacuum suction.
Polishing of the wafer W is performed as follows. The top ring 5 and the polishing table 2 are rotated in the same direction as indicated by arrows, while a polishing liquid (or slurry) is supplied from a polishing liquid supply nozzle 7 onto the polishing pad 1 that is being rotated together with the polishing table 2. In this state, the top ring 5, holding the wafer W on its lower surface, is lowered to a predetermined position (i.e., a predetermined height), at which the top ring 5 presses the wafer W against the polishing surface 1a of the polishing pad 1. A surface of the wafer W is polished by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of the polishing liquid.
FIG. 2 is a schematic view showing a structure of the top ring 5. The top ring 5 includes a plurality of pressure chambers 10 for pressing the wafer W against the polishing pad 1. These pressure chambers 10 are formed by an elastic membrane (or a membrane) 11. A gas, such as an air or a nitrogen gas, is supplied to the pressure chambers 10 separately via a plurality of pressure regulators 15 and a rotary joint 14. Pressures of the gas in the pressure chambers 10 are regulated by the pressure regulators 15. The top ring 5 having such multiple pressure chambers 10 can press multiple zones of the wafer W against the polishing pad 1 at desired pressures.
According to Preston's law, a polishing rate (which is also referred to as a removal rate) of the wafer W is expressed byRR∝P·V where RR represents the polishing rate, P represents a surface pressure exerted on the wafer W when pressed against the polishing pad 1, and V represents a relative speed between the wafer surface and the surface of the polishing pad.
In order to uniformly polish the wafer W, it is preferable that the relative speed V be uniform in the wafer surface. A condition for realizing the uniform relative speed V is that a rotational speed of the polishing table 2 (i.e., a rotational speed of the polishing pad 1) is equal to a rotational speed of the top ring 5 (i.e., a rotational speed of the wafer W).
However, when the wafer W is polished while the polishing table 2 and the top ring 5 are rotated at the same rotational speed, concentric patterns appear on a polished surface of the wafer W. Such appearance of the concentric patterns indicates that the polished surface of the wafer W is not planar. A known solution for preventing such appearance of the patterns is to rotate the polishing table 2 and the top ring 5 at slightly different rotational speeds.
As a demand for a film-thickness uniformity of a wafer has been increasing in recent years, a stability of the pressure in each pressure chamber 10 during polishing of the wafer is becoming more important. In particular, fluctuation of the pressure in the pressure chamber 10 and fluctuation of flow rate of the gas flowing into the pressure chamber 10 are thought to be impediments to the film-thickness uniformity of the wafer.
The surface of the polishing pad 1 is not completely flat. In addition, the top ring 5 may vibrate periodically with the rotation itself. As a result, during polishing of the wafer, a volume of each pressure chamber 10, i.e., the pressure in each pressure chamber 10, may slightly fluctuate with the rotations of the polishing table 2 and the top ring 5. The pressure regulator 15 is operable to cancel the fluctuation of the pressure in the pressure chamber 10 so that the pressure in the pressure chamber 10 is maintained at a predetermined target value. Therefore, a responsiveness of the pressure regulator 15 is important for achieving good polishing results.
FIG. 3 is a schematic view of the pressure regulator 15. The pressure regulator 15 includes a pressure-regulating valve 16 for regulating pressure of the gas in the pressure chamber 10, a pressure gauge 17 for measuring pressure of the gas at a downstream side of the pressure-regulating valve 16 (i.e., outlet pressure or secondary pressure), and a valve controller (for example, a PID controller) 21 for generating a valve control signal for minimizing a difference between a measured value Pact of the pressure and a target value Pc of the pressure. The outlet pressure corresponds to the pressure in the pressure chamber 10. The pressure-regulating valve 16 regulates the outlet pressure according to the valve control signal. An electropneumatic regulator is widely used as the pressure regulator 15.
FIG. 4 is a graph showing a relationship between time and the target value Pc inputted into the pressure regulator 15. As shown in FIG. 4, typically, the target value Pc of the pressure in the pressure chamber 10 is inputted to the valve controller 21 of the pressure regulator 15 at a certain time t0, and is then kept constant. FIG. 5 is a graph showing the actual pressure Pact measured by the pressure gauge 17. As shown in FIG. 5, the pressure Pact reaches the target value Pc after a delay of Δt from the input time t0 of the target value Pc. The pressure Pact after it has reached the target value Pc fluctuates with a certain range ΔP.
From a viewpoint of the responsiveness of the pressure regulator 15, it is desirable that the time difference Δt be as small as possible. In addition, from a viewpoint of the stability of the pressure in the pressure chamber 10, it is desirable that the fluctuation range ΔP be as small as possible. It is possible to improve the responsiveness of the pressure regulator 15 (i.e., to reduce the time difference Δt) by changing an operation setting of the valve controller 21. However, the improvement of the responsiveness may cause, as shown in FIG. 6, an overshoot of the pressure Pact when the target value Pc is inputted, resulting in unstable pressure Pact. On the other hand, in order to prevent the overshoot so as to stabilize the pressure Pact, it is necessary to increase a response time of the pressure regulator 15. However, this means, as shown in FIG. 7, an increase in the time difference Δt.
It is required for the valve controller 21 to improve the response to the input of the target value Pc, eliminate the overshoot, and stabilize the actual pressure Pact corresponding to the constant target value Pc. FIG. 8 is a graph showing a frequency response characteristic of the valve controller 21. A vertical axis in FIG. 8 represents amplification factor which is a ratio of the actual pressure (actually measured value) Pact to the target value Pc. The amplification factor is expressed with use of decibel [dB] as unit. Specifically, in a case where the ratio of the actual pressure Pact to the target value Pc (Pact/Pc) is 1, i.e., in a case where the actual pressure Pact is equal to the target value Pc, the amplification factor is 0 dB. Generally, an ideal amplification factor is 0 dB. The graph in FIG. 8 shows the frequency response characteristic for realizing the responsiveness shown in FIG. 5.
A horizontal axis in FIG. 8 represents frequency of an input control signal inputted into the valve controller 21. The input control signal includes not only the pressure target value Pc, but also the difference between the pressure target value Pc and the pressure value Pact which is a feedback value. The pressure target value Pc is constant as shown in FIG. 4, while the pressure value Pact slightly fluctuates periodically with the rotations of the polishing table 2 and the top ring 5. As a result, the input control signal also fluctuates. The frequency of the input control signal corresponds to an oscillation frequency of the pressure value Pact, and this oscillation frequency of the pressure value Pact corresponds to a frequency calculated from the rotational speeds of the polishing table 2 and the top ring 5. The horizontal axis in FIG. 8 represents this frequency of the fluctuating input control signal (i.e., the frequency of the measured pressure value Pact). A term “fc” shown in FIG. 8 is a resonance frequency.
Generally, during polishing of the wafer, the polishing table 2 and top ring 5 are independently rotated within a speed range of 60 min−1 to 120 min−1. As described above, the surface of the polishing pad 1 is not completely planar, and the top ring 5 may periodically vibrate with the rotation itself. Accordingly, during polishing of the wafer, the volume of the pressure chambers 10 slightly fluctuates with the rotations of the polishing table 2 and the top ring 5. Therefore, a volume Q of a gas storage space including the pressure chambers 10, i.e., a total of the volume of the pressure chambers 10 and a volume of gas passages 28 from the pressure regulators 15 to the pressure chambers 10, fluctuates.
Such fluctuation of the volume Q of the gas storage space affects the pressure value Pact indicating the outlet pressure of the pressure regulator 15, and as a result the input control signal fluctuates at frequency in synchronization with the rotational speeds of the polishing table 2 and top ring 5. For example, when the polishing table 2 and top ring 5 are rotated at the same rotational speed of 60 min−1, the input control signal oscillates at 1 Hz (60 min−1/60 sec=1 Hz). When the polishing table 2 and the top ring 5 are rotated at the same rotational speed of 120 min−1, the input control signal oscillates at 2 Hz (120 min−1/60 sec=2 Hz).
However, as can be seen from the graph in FIG. 8, the amplification factor is not 0 dB (a rate of 1) when the frequency of the input control signal is in a range of 1 to 2 Hz. This indicates that the pressure Pact divergently oscillates when the polishing table 2 and the top ring 5 are rotated at the rotational speed within the speed range of 60 min−1 to 120 min−1.
One of solutions for preventing the divergent oscillation of the pressure Pact is to rotate the polishing table 2 and the top ring 5 at different rotational speeds. When the polishing table 2 and the top ring 5 are rotated at different rotational speeds, the volume Q of the gas storage space (i.e., the pressure chambers 10 and the gas passages 28) fluctuates while greatly undulating with time, as shown in FIG. 9. In FIG. 9, a period T1 of the undulation of the volume Q corresponds to a period that is converted from a difference (which is an absolute value) between the rotational speed of the polishing table 2 and the rotational speed of the top ring 5. A period T2 of the oscillation of the volume Q corresponds to a period that is converted from the rotational speed of the polishing table 2.
As can be seen from FIG. 9, a fluctuation range ΔQ of the volume Q periodically approaches zero. Therefore, the pressure Pact does not divergently oscillate despite the amplification factor of more than 0 dB. As a result, the pressure Pact is maintained at a value close to the target value Pc while the pressure Pact is fluctuating slightly. However, as described above, in order to polish a wafer uniformly, it is desirable to rotate the polishing table 2 and the top ring 5 at the same rotational speed.
Recently, the pressure regulator 15 is typically disposed near the top ring 5, from viewpoints of the responsiveness of the pressure regulator 15 (for reducing the time difference Δt) and an accessibility when exchanging the pressure regulator 15. Accordingly, the volume Q of the gas storage space (the pressure chambers 10 and the gas passages 28) tends to be small. As the volume Q becomes smaller, the fluctuation range ΔQ of the volume Q becomes relatively large. As a result, the volume fluctuation of the pressure chambers 10 greatly affects the pressure Pact.
Further, in a polishing apparatus including multiple sets of polishing tables and top rings, lengths of gas passages may be different between the top rings. Such a difference in the lengths of the gas passages may result in a difference in magnitude of the fluctuation of the pressure Pact between the top rings. As a result, a wafer polishing result may vary between the top rings.
As shown in FIG. 2, the top ring 5 has, at its lower portion, the pressure chambers 10 formed by the elastic membrane (or membrane) 11. The pressurized gas is supplied into these pressure chambers 10. A polishing pressure on the wafer W when pressed against the polishing pad 1 is controlled by the pressures in the pressure chambers 10. The pressures in the pressure chambers 10 are regulated by the pressure regulators 15.
The top ring 5 having the pressure chambers 10 can apply uniform pressure to the wafer in its entirety, as compared with other type of top ring which is designed to press a wafer with a rigid element. Therefore, uniform and stable polishing characteristics can be obtained. However, with a trend toward a high density of devices, the polishing apparatus is increasingly required to have a more improved polishing performance. In particular, there is a strong demand for stabilizing the gas pressures in the pressure chambers 10 during polishing of the wafer.
As shown in FIG. 2, since the polishing table 2 and the top ring 5 are rotated during polishing of the wafer, a relative position in a vertical direction between the polishing table 2 and the top ring 5 is slightly varied with the rotations thereof. Since the interior volumes of the pressure chambers 10 fluctuate with the variation in this relative position, the pressures in the pressure chambers 10 also fluctuate.
Typically, the rotational speeds of the polishing table 2 and the top ring 5 are about 50 to 100 revolutions per minute. Further, the polishing table 2 and the top ring 5 are rotated at different rotational speeds. The reason for this is to prevent the wafer, held by the top ring 5, from sweeping across the same portion of the polishing pad 1 to thereby prevent polishing patterns from occurring on a polished surface of the wafer. In this manner, since there exists a difference in the rotational speed between the polishing table 2 and the top ring 5, the magnitude of the fluctuation of the interior volumes of the pressure chambers 10 also fluctuates.
The interior volumes of the pressure chambers 10 fluctuate with a period corresponding to the rotational speeds of the polishing table 2 and the top ring 5. However, it is difficult for the pressure regulators 15 to follow such instantaneous pressure fluctuation that continuously varies in its magnitude.